Firing rates of motor units during strong dynamic contractions

Muscle behavior is usually studied during isometric contractions but many tasks include contractions that involve changes in muscle length. Our aim was to record motor unit action potentials and surface electromyograms (EMGs) from triceps brachii muscles during rhythmic dynamic contractions (3‐s concentric, 3‐s eccentric; 40°/s; four subjects) performed at the highest voluntary forces subjects could exert (maximal concentric contraction) and at various submaximal intensities. Mean unit firing rates and surface EMG increased significantly with contraction intensity in both concentric and eccentric contractions, but at each intensity mean concentric values were significantly higher than eccentric values. In contrast, mean unit firing rates and surface EMGs were similar during maximal concentric and maximal isometric contractions. These data suggest muscles were activated maximally during the strongest concentric contractions but submaximally during the strongest eccentric efforts. After estimated eccentric contraction intensity was adjusted using surface EMG data, mean unit firing rates during eccentric contractions were still lower than the concentric values. Thus, protective mechanisms may limit motor unit firing rates during forceful lengthening contractions to minimize damage. Muscle Nerve, 2005

[1]  Roberto Merletti,et al.  The extraction of neural strategies from the surface EMG. , 2004, Journal of applied physiology.

[2]  B. C. Abbott,et al.  The effects of force and speed changes on the rate of oxygen consumption during negative work , 1953, The Journal of physiology.

[3]  J. Wessberg,et al.  Single motor unit activity in relation to pulsatile motor output in human finger movements , 1999, The Journal of physiology.

[4]  Benoni B. Edin,et al.  Single unit retrieval in microneurography: a microprocessor-based device controlled by an operator , 1988, Journal of Neuroscience Methods.

[5]  P V Karpovich,et al.  Isotonic and isometric forces of forearm flexors and extensors. , 1966, Journal of applied physiology.

[6]  A. Vallbo,et al.  Somatosensory, proprioceptive, and sympathetic activity in human peripheral nerves. , 1979, Physiological reviews.

[7]  Shi Zhou,et al.  Motor unit synchronisation is enhanced during slow lengthening contractions of a hand muscle , 2002, The Journal of physiology.

[8]  O. Lippold,et al.  Motor unit activity in the voluntary contraction of human muscle , 1954, The Journal of physiology.

[9]  O. Lippold,et al.  The relation between force, velocity and integrated electrical activity in human muscles , 1954, The Journal of physiology.

[10]  B. C. Abbott,et al.  The physiological cost of negative work , 1952, The Journal of physiology.

[11]  J. B. Leeper,et al.  Voluntary activation of human elbow flexor muscles during maximal concentric contractions , 1998, The Journal of physiology.

[12]  A J Fuglevand,et al.  Motor unit activity during isometric and concentric-eccentric contractions of the human first dorsal interosseus muscle. , 1995, Journal of neurophysiology.

[13]  R. Person,et al.  Discharge frequency and discharge pattern of human motor units during voluntary contraction of muscle. , 1972, Electroencephalography and clinical neurophysiology.

[14]  S J Day,et al.  Experimental simulation of cat electromyogram: evidence for algebraic summation of motor-unit action-potential trains. , 2001, Journal of neurophysiology.

[15]  A Thorstensson,et al.  Effects of electrical stimulation on eccentric and concentric torque-velocity relationships during knee extension in man. , 1990, Acta physiologica Scandinavica.

[16]  B. Calancie,et al.  Human motor-unit recruitment during isometric contractions and repeated dynamic movements. , 1987, Journal of neurophysiology.

[17]  C. D. De Luca,et al.  Motor unit control properties in constant-force isometric contractions. , 1996, Journal of neurophysiology.

[18]  K. J. Miller,et al.  Motor-unit behavior in humans during fatiguing arm movements. , 1996, Journal of neurophysiology.

[19]  R. Enoka,et al.  Motor unit physiology: Some unresolved issues , 2001, Muscle & nerve.

[20]  Christine K. Thomas,et al.  Motor unit firing rates during isometric voluntary contractions performed at different muscle lengths. , 2004, Canadian journal of physiology and pharmacology.

[21]  H. Clamann,et al.  Elsevier/North-Holland Biomedical Press COMPARISON OF THE RECRUITMENT AND DISCHARGE PROPERTIES OF MOTOR UNITS IN H U M A N BRACHIAL BICEPS AND A D D U C T O R POLLICIS D U R I N G ISOMETRIC CONTRACTIONS , 2018 .

[22]  O. Kiehn,et al.  Prolonged firing in motor units: evidence of plateau potentials in human motoneurons? , 1997, Journal of neurophysiology.

[23]  L Finsen,et al.  Motor control and kinetics during low level concentric and eccentric contractions in man. , 1996, Electroencephalography and clinical neurophysiology.

[24]  Kazunori Yasuda,et al.  Behavior of single motor units of human tibialis anterior muscle during voluntary shortening contraction under constant load torque , 1985, Experimental Neurology.

[25]  D. Newham,et al.  Voluntary activation of human quadriceps during and after isokinetic exercise. , 1991, Journal of applied physiology.

[26]  A. Kossev,et al.  Discharge rate of selected motor units in human biceps brachii at different muscle lengths. , 1998, Journal of electromyography and kinesiology : official journal of the International Society of Electrophysiological Kinesiology.

[27]  G. L. Soderberg,et al.  The effect of muscle length on motor unit discharge characteristics in human tibialis anterior muscle , 2004, Experimental Brain Research.

[28]  S. Gandevia,et al.  Discharge properties and recruitment of human diaphragmatic motor units during voluntary inspiratory tasks , 1999, The Journal of physiology.

[29]  A. del Valle,et al.  The role of motor unit rate modulation versus recruitment in repeated submaximal voluntary contractions performed by control and spinal cord injured subjects. , 2001, Journal of electromyography and kinesiology : official journal of the International Society of Electrophysiological Kinesiology.

[30]  R. Enoka,et al.  Steadiness is reduced and motor unit discharge is more variable in old adults , 2000, Muscle & nerve.

[31]  A. Monster,et al.  Isometric force production by motor units of extensor digitorum communis muscle in man. , 1977, Journal of neurophysiology.

[32]  J Tanji,et al.  Firing rate of individual motor units in voluntary contraction of abductor digiti minimi muscle in man. , 1973, Experimental neurology.

[33]  S C Gandevia,et al.  Voluntary discharge frequencies of human motoneurons at different muscle lengths , 1992, Muscle & nerve.

[34]  B. Katz.  The relation between force and speed in muscular contraction , 1939, The Journal of physiology.

[35]  B Bigland-Ritchie,et al.  Motor-unit discharge rates in maximal voluntary contractions of three human muscles. , 1983, Journal of neurophysiology.

[36]  A Kossev,et al.  Discharge pattern of human motor units during dynamic concentric and eccentric contractions. , 1998, Electroencephalography and clinical neurophysiology.